Color electrophotographic process employing a polar organic solvent vapor

ABSTRACT

IN A COLOR ELECTROPHOTOGRAPHIC PROCESS OF THE REGISTRATION DEVELOPMENT TYPE WHRE AN IMAGE BEARING PHOTOCONDUCTIVE LAYER COMPRISING A PARTICULATE PHOTOCONDUCTIVE MATERIAL DISPERSED IN A THERMOSETTING RESIN IS SUBJECTED TO REPEATED SEQUENCING WHERE EACH SEQUENCING COMPRISES AT LEAST THE STEPS OF ELECTROSTATIC CHARGING, IMAGE EXPOSURE TO LIGHT AND DEVELOPMENT, THE IMPROVEMENT COMPRISING THE STEPS OF REDUCING LIGHT FATIGUE BETWEEN EACH SEQUENCING ON THE PHOTOCONDUCTIVE LAYER BY EXPOSING THE LAYER TO VAPORS OF A POLAR ORGANIC SOLVENT AFTER EACH SEQUENCING EXCEPT THE LAST OF THE SEQUENCINGS, THE POLAR ORGANIC SOLVENT HAVING A MINIMUM SPECIFIC DIELECTRIC CONSTANT OF AT LEAST 5 TO 20*C., AND MAINTAINING THE INTENSITY OF THE ELECTROSTATIC CHARGING STEPS SO THAT THE INTENSITY OF ANY ONE CHARGING STEP IS NOT GREATER THAN APPROXIMATELY THAT OF A PREVIOUS CHARGING STEP.

United States Patent US. Cl. 96-12 17 Claims ABSTRACT OF THE DISCLOSURE In a color electrophotographic process of the registration development type where an image bearing photoconductive layer comprising a particulate photoconductive material dispersed in a thermosetting resin is subjected to repeated sequencing where each sequencing comprises at least the steps of electrostatic charging, image exposure to light and development, the improvement comprising the steps of reducing light fatigue between each sequencing on the photoconductive layer by exposing the layer to vapors of a polar organic solvent after each sequencing except the last of the sequencings, the polar organic solvent having a minimum specific dielectric constant of at least 5 at 20 C., and maintaining the intensity of the electrostatic charging steps so that the intensity of any one charging step is not greater than approximately that of a previous charging step.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to an improvement in registration development type color electrophotography.

(2) Description of the prior art Registration development type color electrophotography is an electrophotographic method known to the art. A typical example of such is described below.

A sheet of white or light gray electrophotographic paper having sensitivity over almost the entire visible region is evenly charged by corona discharge in the dark and is exposed to a color image through a green filter, thereby forming an electrostatic latent image thereon. This latent image can be developed into a visible magenta image with a magenta toner. Subsequently, the electrophotographic paper is evenly charged with electricity in the dark and is exposed to the same color image through a red filter in registration with the said magenta image, thereby forming a latent image thereon. This latent image is then developed with a cyan toner. The electrophotographic paper is then charged once again in the dark and is exposed to the same color image through a blue filter to form a latent image. This latent image is developed with a yellow toner, to reproduce the color image. This overall method of reproducing a color image is known as registration development type electrophotography.

In the described example, exposure to the color image was in the order green, red and blue. Such a registration development type color electrophotography has certain drawbacks as will be hereinafter explained. The main drawback of such a process is encountered when an electrophotographic paper with the toner image of the first color developed thereon is evenly charged during the second step of the imaging operation. More particularly, the exposure to the color image for the formation of the latent image of the first color causes light fatigue of the electrophotographic paper, which will remain therein to affect the second charging. Due to this residual light fatigue, the second charging must be more intense than the first charging. More specifically, without more intense charging, it would be impossible to provide the electrophotographic paper with a surface potential equal to that by the first charging. More intense charging, in connection with the practical charging mode involving corona discharge, comprises bringing the electrophotographic paper closer to the corona discharge electrode than when it was charged for the first time. Of course, an increased voltage can be applied to the color discharge electrode, or else the color discharge electrode can be passed over the surface of the electrophotographic paper an increased number of times.

Such intensified electric charging, however, has various harmful effects. For instance, it causes dielectric breakdown at the surface of electrophotographic paper to such an extent that pin holes in the form of dots more readily occur thereon. These pin holes are not electrically charged, and result in blank spots in the toner image. In addition, the toner image of the first color is subjected to an unnecessary over electrical charging. Such charge in the toner image is diflicult to discharge completely even by the second exposure, causing an undesirable color mixture or impure color when the toner image of the second color is developed.

For those cases where registration development type color electrophotography is carried out quickly, it is difficult to develop the electrostatic latent image to the point where the electric charge thereof is completely eliminated. More particularly, rapid processing shortens the developing time, as a result of which some of the latent image of the first color remains to be developed completely thereby leaving a certain amount of residual static charge. This residual static charge present in the toner image of the first color causes unnecessary adhesion of the toner for the second color as a result of the second development, which also results in a color mixture.

SUMMARY OF THE INVENTION The present invention essentially comprises a color electrophotographic technique characterized by exposing an electrophotographic paper surface bearing a toner-developed image to a polar solvent vapor for a comparatively short period of time in the aforementioned registration development type color electrophotography be tween development and subsequent charging processes.

The inventors have found that by the exposure of electrophotographic paper surfaces bearing a toner-developed image to a polar solvent vapor in the dark, light fatigue thereof can be avoided to thereby facilitate subsequent electric charging. It has further been found that the unused static charge left in the toner image can be eliminated completely.

One object of the present invention is to overcome the problem of electrophotographic paper light fatigue caused by rapid exposure in the registration development type color electrophotography.

Another object is to eliminate the residual charge left in the electrophotographic paper under the toner image after development.

3 DETAILED DESCRIPTION OF THE INVENTION Experiments have shown that a polar organic solvent having a melting point below room temperature, a boiling point of 100 C. or less and a specific dielectric constant of 5 or greater at 20 C. can be used as the polar solvent of this invention. It is important that the specific dielectric constant of the solvent be no lower than 5. This requirement is supported by the fact that trichloroethylene (with a specific dielectric constant of 3.42), often utilized in the electrophotographic solvent vapor fixing process, has no effect in permitting the recovery from light fatigue as in the process of the present invention.

80 long as a polar organic solvent meets the above criteria it may be used in the present invention. Acceptable polar organic solvents can easily be determined by one skilled in the art with a preliminary process run. The preferred polar organic solvents have a maximum dielectric constant at 20 C. of about 35, though polar organic solvents having a dielectric constant above this value can, of course, be used.

Solvents having a boiling point above 100 C. are difficult to vaporize and are of low vapor pressure at normal temperature. These may therefore be retained in the photoconductive sheet for rather long periods of time after treatment, which is undesirable.

Organic solvents having such properties include alcohols, such as methanol and ethanol, ketones, such as acetone and methylethyl ketone, and esters, such as methyl acetate, ethyl formate and propyl formate.

The process of this invention should not be confused with methods of fixing a toner image by solvent vapors which methods involve fixing a toner image by adherance of a thermoplastic resin component contained in the toner, a photosensitive layer or in the transfer paper. For instance, U.S. Pats. 2,726,166, 2,922,230, 2,995,464, 3,013,- 324, 3,028,683, 3,049,810 and 3,140,159, etc., relate to the solvent vapor fixing method wherein a final image is obtained by fixing the toner image onto the photosensitive paper or the transfer paper.

On the contrary, the light-fatigue recovery (effect according to the present invention proved to depend on the polarity of the solvent used.

The electrophotographic paper used in the process of the present invention is provided with a photosensitive layer composed of mixture of photoconductive powder and a resin binder on the surface thereof. A thermosetting binder resin is preferred, as electrophotographic papers using a thermosetting resin binder are well suited to liquid development processing, and a thermosetting binder has the advantage of not being affected by the polar solvent vapors.

Thermoplastic resins are not preferred as binders for the photosensitive layer, as they tend to swell or soften to such a degree when used with the polar solvents of this invention that further treatments become difilcult, or special care has to be taken during further treatments.

The photoconductive material used in the present invention includes those well known in the art, and no special novelty is attached to the photoconductive material per se. Such materials may comprise the essential photoconductive material per se or such material may be modified with the many special sensitizing agents, additive materials, etc., known to the art. Many patents are available describing such materials, e.g., US. Pat. 3,121,006, etc.

The preferred photoconductive materials employed in the present invention are photoconductive zinc oxide, titanium oxide, cadmium sulfide, zine sulfide, etc. Zinc oxide produced by the French process (firing zinc in air to produce ZnO) is most preferred. By the use of a photoconductive substance sensitized to the entire visible region, a color image is directly obtainable from a color positive or negative original by color separation exposure.

Thermosetting binder resins which are typically employed in the process of this invention include polyisocyanate-cured alkyd resins, epoxy ester resins and copolymers of a vinyl monomer having a primary OH group (hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, etc.) and styrene, acrylic and methacrylic esters or the like, alkyd resins cured with melamine formaldehyde resins, alkyd resins cured with benzoguanamine formaldehyde resins, drying oil modified alkyd resins, epoxy ester resins cured with a catalyst e.g., salts of organic acids, such as cobalt, lead and manganese salts thereof, etc.

Various electrophotographic development techniques may be used in the present invention, of which liquid development methods are preferred, most preferably wherein a liquid developer containing a toner composed primarily of pigment is used. Other suitable development techniques include, for example, aerosol development, cascade development or magnetic development (cf. U.S. Pats. 2,725,304, 2,786,441, 2,618,551 etc.).

Conventional liquid developers comprising a dispersion of a pigment and an alkyd resin, expoxy ester, acrylic ester, polystyrene, etc., in a carrier liquid may be suitably employed in the present invention, e.g., see, for example, US. Pats, 3,053,688, 3,076,722, 2,907,674, etc.

The exposure of the electrophotographic paper with the toner image of the first color thereon to a polar solvent vapor can be accomplished by any desired technique, e.g., by blowing air containing the vapor onto the photosensitive layer side of the electrophotographic paper or by placing the electrophotographic paper in a container filled with the vapor. Depending on the composition of the photosensitive layer involved, exposure to the vapor for about one second to about one minute is preferred.

Treatment for more than one minute proved to cause 510 further advantageous effect on the photoconductive ayer.

As it is clear from the heretofore offered explanation, the treatment with the polar solvent vapor is repeated twice for a three-color imaging process (magenta, cyan and yellow, for example), three times for a four-color imaging process (magenta, cyan, yellow and black, for example). The difference in number between exposures to the vapor and the colors formed is due to the fact that vapor treatment is not necessary when the final color image has been obtained.

As has been explained, the process of the present invention is effective for quickly eliminating residual charge and the hysteresis effect results which result from previous exposures. This means that it not only finds particularly effective use in registration development type color electrophotography but also extensive application in all kinds of electrophotographic processes wherein a photoconductive layer is acclimatized to light by a previous exposure. Such a layer may be brought into the dark where it is exposed to polar solvent vapors, e.g., acetone or metha- 1101, according to the process of the present invention, whereby it quickly recovers from fatigue and is restored to its dark-acclimatized state.

conventionally, the recovery of light-sensitive layers from light-fatigue has been effected by means of heating or infrared dariation. If an object has large heat capacity, e.g., if the electrophotography is applied to the marking of a metal plate, a large quantity of energy is required, making such heat restorations practically very difficult. The present invention offers particular advantages in such applications. The following examples will describe several specific embodiments of the present invention:

EXAMPLE 1 (INCLUDING A COMPARATIVE EXAMPLE) Five weight parts of photoconductive zinc oxide powder having added thereto tartrazine for sensitization of the blue region (430-500 mg), erythorosine for the sensitization of the green region (520-580 mg) and Brilliant Milling Green B (0.1. Acid Green 9) for the sensitization of the red region (630-680 mg), at a ratio of mg. to 10 g. of the zinc oxide, respectively, was dispersed in one weight part of a composition of a styrenated alkyd resin (the styrenated alkyd resin was a commercial product named Styresol 4250 by Japan Reichhold Chemical Co.,) and a polyisocyanate compound curing agent (the polyisocyanate was a condensation product of 3 moles of tolylene diisocyanate and 1 mole of trimethylol propane) to cure the styrenated alkyd resin (mixed at ratio of 0610.4).

The dispersion was applied to a sheet of paper made electrically conductive (Surface resistivity of the paper was about 2X10 ohm per square cm. at 55% RH) by the immersion in a solution of a hygroscopic substance. The dried thickness of the photosensitive layer was 10 microns.

The hygroscopic substance was poly(vinyl benzyl trimethyl ammonium chloride) incorporated into the paper in an amount of 2 g. per m?.

The photosensitive paper thus prepared Was stored in the dark for a period of time sufficient to have the photosensitive layer thereof acclimatized to the darkness. Two corona discharge electrodes (impressed with 6 kv.) were then passed 3 cm. above the photosensitive paper to charge the photosensitive layer surface. Upon sweeping the corona discharge electrodes three times, over the surface of the paper the photosensitive layer was charged to a surface potential of 270 v. The photosensitive layer surface was then exposed to imaging light projected thereon through a color slide color positive original and a red filter.

The resulting electrostatic latent image was developed by the use of a cyan developer prepared as explained hereinafter containing a toner with positive charge. The development lasted for seconds under the influence of the development electrode, which was maintained at ground potential. Immediately thereafter, the photosensitive layer was washed with pure isooctane and dried, whereupon the surface potential thereof was (residual potential) -15 v. in the cyan toner-deposited area (a subsequent optical density measurement showed that this area had an image density of 1.90).

Subsequently, the photosensitive layer was exposed to methanol vapor in a container, which eliminated the residual potential in four seconds. After a further eleven seconds retention in the container, the photosensitive layer was subjected to the repeated charging process in the heretofore described manner. Charging with three sweeps of the corona discharge electrodes resulted in a toner-free area electrically charged to a surface potential of -261 v. The toner-deposited area showed a potential of 265 v.

For comparative purposes an identical photosensitive layer was not exposed to methanol vapor but rather was permitted to remain in the air for fifteen seconds following the development process. This element was electrically charged as heretofore described. As a result the tonerfree area of the photosensitive layer was found to have a potential of only 102 v.

The photosensitive layer electrically charged to 26 v. in accordance with the present invention was exposed to imaging light through a green filter. The resulting electrostatic latent image was developed for fifteen seconds by the use of magenta developer prepared as described hereinafter containing a toner with a positive charge. It was then washed with pure isooctane and dried. In the area wherein magenta toner deposited there was found a potential of v. (a subsequent optical density measurement showed that this was an area having an image density of 1.86). This residual potential was completely eliminated by exposure to methanol vapor for five seconds. A subsequent ten second exposure to the methanol vapor was followed by charging once again in the aforementioned manner.

The toner-free area of the photosensitive layer was electrically charged to -258 v. In addition a comparative photosensitive layer was electrically charged by the same corona discharge electrode sweeping five times immediately after development with the cyan developer. As a result, the toner-free area was charged to -250 v. and the tonerdeposited area was chraged to -290 v. This photosensitive layer (not treated with methanol vapor) was developed after being to imaging light in the same manner as the methanol vapor treated element to find that the magenta toner-deposited area (having an optical density of 1.80) showed a residual potential of 18 v. This photosensitive layer was charged after being left in the air for fifteen seconds, and the toner-free area was charged to only 35 v. with three sweeps. Charging with five sweeps charged it to a potential of 225 v.

After exposure to imaging light through a blue filter, the aforementioned methanol vapor-treated photosensitive layer charged to -25 8 v. It was thereafter developed for fifteen seconds by the use of a yellow developer prepared as described hereinafter containing a toner with positive charge. The comparative photosensitive layer charged to '-225 v. was also exposed and developed in the same manner. The only difference (other than the use of 3 versus 5 corona discharge sweeps) in the above proc- At charging alter- Betore Cyan Magenta Corona esposing developdevelopcharging to red, merit merit, operation volts volts volts Example according Three times--- 270 -261 -258 to the present invention. Comparative {Three times... 270 --102 35 example. Five times -250 -225 In the example according to the present invention, the surface potential at each step is approximately the same using the same charging operation. However, in the comparative example, stronger charging is necessary at the second and the third charging steps to obtain a sufficient surface potential. This illustrates that the photosensitive layer is fatigued by light, and that treatment with a polar solvent vapor, e.g., methanol vapor, is effective to revive the photosensitive layer.

A comparison of the final color images obtained showed that the photosensitive layer subjected to two methanol vapor treatments provided a color image of better quality with little color mixture, whereas a pinholed image with many blank spots and impure tone resulted on the photosensitive layer not treated with methanol vapor. The blank spots and pinholes can be attributed to dielectric breakdown of the photosensitive layer caused by charging with an increased number of corona discharge electrode sweeps. The color mixture is due to the residual charge left on the toner deposited area after development. Methanol with a boiling point of 641 C. and a dielectric constant of 33.2 was employed.

(I) As the cyan developer, a mixture with the following composition was utilized.

Weight parts Phthalocyanine Blue 1 Long-oil type safilower oil modified alkyd resin 3 Kerosene 500 (II) As the magenta developer, a mixture containing 1.2 weight parts of Brilliant Carmin Blue 6B in place of the Phthalocyanine Blue contained in the cyan developer was utilized.

(III) As the yellow developer, a-mixture containing 1.2 weight parts of Benzidine Yellow in place of Phthalocyanine Blue contained in the cyan developer was utilized.

The long-oil type safflower oil modified alkyd resin used in this example, whose oil length was 72%, is soluble in isooctane, but not in methanol. It is thus clear that solvent vapor fixing does not accrue by treating with methanol vapor.

EXAMPLE 2 The procedure of Example 1 was duplicated using methyl acetate vapor instead of the methanol vapor utilized in Example 1. It was found that methyl acetate was slightly inferior to methanol in its ability to recover the photosensitive layer from light fatigue. Methyl acetate with a boiling point of 57.5 C. and a specific dielectric constant of 6.68 was used.

EXAMPLE 3 The procedure of Example 1 was duplicated utilizing acetone vapor instead of methanol vapor, to find that acetone vapor was almost equal in effect to methanol vapor. The acetone used had a 56.5 C. boiling point and a specific dielectric constant of 21.4.

For comparison, Example 1 was duplicated except for using trichloroethylene instead of methanol. Trichloroethylene failed to show the intended effect of the present invention at all, and gave results the same as the case where the photosensitive paper was left in air.

The trichloroethylene used had a 89 C. boiling point and a specific dielectric constant of 3.42.

EXAMPLE 4 A photosensitive layer was prepared exactly as in Ex ample 1 using a methanol modified benzoguanamine resin. (The resin was BB-601 a trade name of the Nippon Shokubai Kagaku Co.) instead of the polyisocyanate compound curing agent. Results similar to those obtained in Examples 1, 2 and 3 involving methanol vapor, methyl acetate vapor and acetone vapor, respectively, were obtained.

EXAMPLE 5 The photosensitive layer used in Example 1 was left in the light. Immediately thereafter, this photosensitive layer was transferred into the dark where it was electrically charged by sweeping two corona discharge electrodes impressed with 6 kv. 3 cm. thereabove three times in the same manner as in Example 1.

The resultant surface potential was only 21 v. On the other hand, an identical photosensitive layer exposed to acetone vapor for thirty seconds in the dark after being kept in the light was electrically charged under the same conditions, resulting in a potential of --160 v.

For comparison, a photosensitive layer left in the light was exposed to air for thirty seconds in the dark and was electrically charged under the same conditions as above, resulting in a surface potential of -30 v. only. This procedure confirmed the advantageous results obtained in accordance with the present invention.

What is claimed is:

1. A color electrophotographic process comprising contacting a photoconductive layer comprising a thermosetting resin binder and a photoconductive material with a polar organic solvent vapor having a specific dielectric constant of at least 5 at 20 C. after the development and before the charging processes on said photoconductive layer in a registration development type color photograph where the process of developing the electrostatic latent image formed on the photoconductive sensitive layer surface with the toner is repeated.

2. A process as in claim 1 where said vapor exposure step is performed in the dark.

3. The process of claim 1 where said contacting with said polar organic solvent vapor is conducted (n-l) times, where n is the number of separate color exposures in said registration type development.

4. The process of claim 1 where said polar organic solvent has a melting point below room temperature and 'a boiling point of C. or less.

5. The process of claim 1 where said polar organic solvent has a specific dielectric constant no greater than about 35 at 20 C.

6. The process of claim 1 where the developing is with a liquid developer.

7. The process of claim 1 where said contacting is for at least about one second.

8. The process of claim 7 where said contacting is for a period ofv no larger than about 1 minute.

9. The process of claim 1 where the polar organic solvent is an alcohol or ketone.

10. In a color electrophotographic process of the registration development type wherein an image bearing photoconductive layer comprising a particulate photoconductive material dispersed in a thermosetting binder is subjected to the repeated sequencing where each sequencing comprises the steps of electrostatic charging, exposure to light of one component color which is to form the final color image, and development, the improvement of reducing light fatigue between each of said sequencings on said photoconductive layer by exposing said layer to vapors of a polar organic solvent for a period of time of from about 1 second to about one minute after each sequencing except the last of said sequencings, said polar organic solvent having a minimum specific dielectric constant of at least 5 at 20 C., a melting point below room temperature and a boiling point of 100 C. or less.

11. A process as in claim 1 where said vapor exposure step is performed in the dark.

12. The process of claim 10 where said polar organic solvent has a maximum specific dielectric constant of about 35.

13. The process of claim 12 wherein said polar organic solvent is an alcohol or a ketone.

14. In a repetitive electrophotographic process wherein an image bearing photoconductive layer comprising a particulate photoconductive material dispersed in a thermosetting resin is subjected to repeated sequencing where each sequencing comprises steps of at least electrostatic image formation and development, the improvement comprising reducing light fatigue between each sequencing on said photoconductive layer by exposing said layer to the vapors of a polar organic solvent after each sequencing except the last of said sequencings, said polar organic solvent having a minimum specific dielectric constant of at least 5 at 20 C.

15. A process as in claim 14 where said vapor exposure step is performed in the dark.

16. In a color electrophotographic process of the registration development type where an image bearing photoconductive layer comprising a particulate photoconductive material dispersed in a thermosetting resin is subjected to repeated sequencing where each sequencing comprises at least the steps of electrostatic charging, image exposure to light and development, the improvement comprising the steps of reducing light fatigue between each sequencing on said photoconductive layer by exposing said layer to vapors of a polar organic solvent after each sequencing except the last of said sequencin s, said polar organic solvent having a minimum specific dielectric constant of at least 5 at 20 C.; and maintaining the intensity of the electrostatic charging steps so that the intensity of any one charging step is not greater than approximatel that of a previous charging step.

, 9 10 17. A process as in claim 16 where said vapor exposure 3,620,800 11/1971 Famai 96-12 step is performed in the dark. 3,032,432 5/1962 Metcalfe et a1 96-1 LY References Cited UNITED OTHER REFERENCES Handbook of Chemistry and Physics, 1967-4968, 48th STAT PATENT ES 5 edition, pp. 15-57 to 5-60.

Weigl a a1. 96-4 R M atsumoto et a1 96-1 LY I. TRAVIS BROWN, Primary Examiner Fischer 117--37 Matkan I I. R. MILLER, Assrstant Exammer Moe et a1 961 0 us. (:1. xx. Bixby 96-12 96-1 R, 1.3 

